The invention relates to radar systems generally but more particularly to planar array, directive radiating and receiving systems in which the plane of polarization of transmitted or received RF energy may be electronically controlled in a substantially inertialess manner.
In the prior art, polarization-agile radar systems are known, per se, in various forms. The use of gratings and lenses in the near field of a radar array are well-known expedients for twisting or rotating a plane of polarization; however, such electromechanical apparatus is not generally regarded as qualifying for polarization agility. Rather, electronically controllable means of the substantially inertialess type are commonly involved in truly agile polarization systems. Since they are often combined with frequency or phase scanning arrangements, it is particularly necessary and important that polarization changes be relatively rapidly accomplished for a number of reasons well understood in the art.
Planar array antennas have come into use in radar systems, because they have advantages of higher gain and lower sidelobes compared with optically fed antennas of conventional design and, moreover, are particularly adaptable to frequency or phase types of inertialess scanning.
Arrays of physical dipoles have been employed in planar arrays involving corporate feeds and slow-wave structures to increase the change of beam pointing angle as a function of frequency or phase change (scan sensitivity).
The prior art in planar and phased arrays is discussed in the text, RADAR HANDBOOK, by Merill I. Skolnick (McGraw-Hill Book Company, 1970), particularly, in Chapter 13, Section 13.3, and in Chapter 11.
The general concept of converting a relative phase shift in energization between first and second interleaved groups of discrete dipole linear arrays forming a planar array in order to produce polarization agility is known; however, it is difficult and even at times impossible to control the interaction among physical dipoles in a planar array. Slotted waveguide arrays, although used in frequency and phase scanned arrays, are not suitable for the application of the differential phase shift concept to produce polarization agility or controllability, because the electrical wavelength within a waveguide is relatively long, causing the radiating slots to be relatively widely spaced, giving rise to grating lobes, unless dielectric loading is employed to decrease the effective physical wavelength. In a waveguide arrangement, such an expedient would be expensive and heavy.
The manner in which the present invention solves the problems of the prior art and addresses the needs for inexpensive and efficient polarization agility in a planar (pencil beam forming) array will be evident as this description proceeds.